And here are my questions:
* How does debian amd64 compares with other distros (suse, fedora) in
terms of stability and 64 bit efficiency?

Usually gentoo guys would disclaim that gentoo is way faster than debian
because of all their fine-tuned compile options specifically tailored to
the system they're building on. Well, this might be true for i386, but
it surely isn't for amd64 - simply because all the debian pure64
packages are built using the amd64 optimizations: there is no
speed-vs-backwards-compatibility tradeoff here. As for stability, debian
pure64 is mainly based on debian sid/unstable. It runs fine for me, but
I probably wouldn't want to run it on production critical systems. But
actually I doubt that's different with the other distributions - amd64
just isn't tested as well as i386 is for obvious reasons ;)

* What is the difference between pure64 and gcc-3.4? I saw somewhere
that gcc-3.4 binaries run faster. Is there a catch?

I haven't tried the gcc-3.4 archive myself, but rumor has it that it is
faster - gcc-3.4 making more aggressive optimizations using the new
registers the amd64 introduced. The catch being that the official debian
ports are running with gcc-3.3. So the gcc-3.4 branch is an amd64
specific one. It is planned to continue that branch, but there's no
estimation as to when all those branches will finally converge...

* The mailing list left me with the impression that sarge is not ready
yet, is it?. Does it keep up with the i386 sarge? Can it be installed
like an i386 sarge (net inst->base system->package installation)?

Well, official sarge is going to be released "soon". amd64 sarge is
actually a symlink to sid, afaik (correct me if I'm wrong). As for
installation, there are debian installer CD images available for amd64.
Generic, i.e. i386, netinstall images wouldn't work.

Thank you for the information.

I get a vague feeling that 64 bit sarge is less supported than pure64.